The present application is generally directed to making pulp and is more specifically directed to screen assemblies for pulp digesters.
Continuous digesters are used in the paper and pulp industry to remove lignin from wood chips. The digesters generally include a series of tubular reactors that are arranged in a vertical orientation. The digester is made up of a plurality of reaction and extraction stages for carrying out the pulp-making process in a specific sequence. In certain stages, chemicals are introduced into the digester for chemically treating the woodchips. These chemicals may include hydrosulfide and sodium hydroxide, commonly referred to as liquor. In other stages, the chemicals are removed from the reactor through screens provided inside the digester.
FIGS. 1A and 1B show a conventional pulp digester 20 having a vessel wall 22 with an interior surface 24 and an exterior surface 26. The vessel 22 has a step out section 28 and a straight section 30 located below the step out section. The step out section 28 diverges outwardly for expanding the diameter of the vessel 22 between an upstream section of the digester and a downstream section of the digester.
The digester 20 includes a bar screen 32 having an upper end 34 and a lower end 36. The bar screen 32 includes a plurality of vertically extending, cylindrical bars 38 that are spaced from one another. The upper end 34 of the bar screen 32 is attached to the vessel via an upper support element 40 and the lower end 36 of the bar screen 32 is attached to the vessel via a lower support element 42. The bar screen is permanently fixed to the vessel wall 22, such as by welding the upper and lower ends 34, 36 of the bar screen 32 to the upper and lower support elements 40, 42. The bar screen 32 has a diameter that remains substantially constant between the upper and lower ends 34, 36 thereof.
The digester also includes an upper cover plate 44 having an upper end 46 and a lower end 48. The upper end 46 of the upper cover plate 44 is secured to the interior surface 24 of the vessel at a location above the step out section 28. The lower end 48 of the upper cover plate 44 is secured to the upper support element 40. The digester also has a lower cover plate 50 having an upper end attached to lower support element 42. The lower cover plate 50 extends downwardly from the bar screen 32 to a downstream section (not shown) of the digester. The digester has an outlet 52 for removing liquid from the vessel.
Permanently fixing the bar screen to the wall 22 involves a significant amount of work when assembling the digester. For example, when welding is used, the manufacturing process requires hundreds of welds to attach one bar screen to the vessel wall. For a vessel having many bar screens, the number of required welds may be in the thousands. The large number of welds increases the chances that one or more of the welds will crack, which may adversely affect operation of The digester or may require the digester to be taken off-line for repairs.
The production of quality pulp involves introducing and removing liquor from the digester at certain time periods. The liquor is typically removed from digesters by passing the liquor through screens in a radial direction. The radial removal of the liquor causes compression of the wood chips onto the screen. This may prevent the wood chips from continuing to move toward the bottom of the digester, which is critical for the proper treatment of the chips. Extreme radial compression may also limit the amount of liquor that can be removed from the digester. Both of these situations may adversely affect the quality of the pulp produced using the digester.
FIGS. 2A and 2B show a pulp digester having a diverging screen assembly. The digester 120 has a vessel wall 122 with an interior surface 124 and an exterior surface 126. The vessel 122 has a step out section 128 and a straight section 130 located below the step out section. The step out section 128 diverges outwardly for expanding the diameter of the vessel 122 between an upstream stage and a downstream stage of the digester.
The digester 120 includes a diverging bar screen 132 having an upper end 134 and a lower end 136. The bar screen 132 includes a plurality of vertically extending, cylindrical bars 138 that are spaced from one another. The upper end 134 of the bar screen 132 is attached to the vessel via an upper support element 140 and the lower end 136 of the bar screen 132 is attached to the vessel via a lower support element 142. The bar screen continuously diverges between upper and lower ends thereof for reducing the compression loading of the wood chips on the screen. The digester also includes an upper cover plate 144 having an upper end 146 and a lower end 148. The upper end 146 of the upper cover plate 144 is secured to the interior surface 124 of the vessel at a location above the step out section 128. The lower end 148 of the upper cover plate 144 is secured to the upper support element 140. The digester also has a lower cover plate 150 having an upper end attached to lower support element 142. The lower cover plate 150 extends downwardly from the bar screen 132 to another downstream stage (not shown) of the digester. The digester has an outlet 152 for removing liquid from the vessel.
In spite of the above advances, there is a need to provide continuous digesters that more efficiently introduce and remove liquor from the digester vessel at various stages of the pulp making process. There also remains a need for simplified methods for building digesters. Specifically, there remains a need for simpler methods for assembling screen assemblies inside digesters that minimize the number of welds needed for securing the screen inside the digester. Further, there remains a need for screen assemblies that are less subject to breakage during digester operations.